The chalicotheres were prehistoric mammals that were distantly related to modern horses, tapirs and rhinoceroses, but were really a branch of their own.
Facts
Reaching on average 3 m in height (the males were more massive than the females were), the chalicotheres were built like modern apes, with forelimbs that were much longer than their hind legs and ended in hook-like claws that they probably used to pull down branches to get at the softest leaves.
Chalicotheres walked on their knuckles to protect their long claws. These powerful limbs were very effective defensive weapons, but more often these herbivores used them to hook down branches to get at the softest leaves. Chalicotheres walked like gorillas and ate like pandas, but their closest modern relatives were actually horses, even though they are still distant relatives.
(It should be noted that some chalicotheres weren't knuckle-walkers, but did move on all fours, as shown by Ancylotherium from the next episode, Next of Kin - this animal was a close relative of the knuckle walking chalicothere from Land of Giants, but it walked on all fours, like a more ordinary mammal.)
Several chalicotheres were featured in the third episode of WWB.
Hyaenodon attacking Chalicotherium.
They were depicted as peaceful herbivores, prey for such carnivores as Hyaenodon and the Entelodonts.
The last of the chalicotheres died out at the beginning of the Pleistocene period (ep. 6 of WWB), but the species depicted in the third episode, Chalicotherium, died out earlier, 7.75 MYA.
There have been suggestions that calicotheres survive to the present day in the form of a cryptid animal called the Nandi bear, but there are many problems with this idea, so they are almost certainly extinct.
An image of Chalicothere (Chalicotherium).